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UNIT-VClassificationof metal forming operations

Cutting of sheet metal is accomplished by a shearing action between two sharp cutting edges. The shearing action is depicted in the four stop-action sketches of Figure , in which the upper cutting edge (the punch) sweeps down past a stationary lower cutting edge (the die).As the punch begins to push into the work, plastic deformation occurs in the surfaces of the sheet.As the punch moves downward, penetration occurs in which the punch compresses the sheet and cuts into the metal.CUTTING OPERATIONS

Fig: Shearing of sheet metal between two cutting edges: (1) just before the punch contacts work;(2) punch begins to push into work, causing plastic deformation; (3) punch compresses and penetrates into workcausing a smooth cut surface; (4) fracture is initiated at the opposing cutting edges that separate the sheet.Symbols v and F indicate motion and applied force, respectively, t =stock thickness, c=clearance.BLANKINGBlanking involves cutting of the sheet metal along a closed outline in a single step to separate the piece from the surrounding stock, as in Figure 20.4(a). The part that is cut out is the desired product in the operation and is called the blank.PUNCHINGPunching is similar to blanking except that it produces a hole, and the separated piece is scrap, called the slug.

BENDINGBending means deforming a flat sheet along a straight line to form the required angle. Various sections like angles, channels etc., are formed by bending, which may then be used for fabrication of steel structures.Three common methods of bending are illustrated in Fig.

The operation of bending is done with the help of a V-shaped punch, a die and press specially designed for such work. The stroke of such presses can be controlled at operators will and such presses are called press brakesTypes of bending diesIn V-bending, a V-shaped punch forces the metal sheet or a flat strip into a wedge-shaped die.The bend angle will depend upon the distance to which the punch depresses. Bends of 90 or obtuse as well at acute angle, may be produced.

Wiper bending is used only for 90 bends. Here the sheet is held firmly down on the die, while the extended portion of sheet is bent by the punchDEEP DRAWINGIn deep drawing process, we start with a flat metal plate or sheet and convert it into cup-shape by pressing the sheet in the centre with a circular punch fitting into a cup shaped die.If the depth of cup is more than half its diameter, the process is termed as deep drawing and with a lesser depth to diameter ratio, it is called shallow drawing.Parts of various geometries and shape are made by drawing process

Deep drawing operationDuring the drawing process, the sheet metal part is subjected to a complicated pattern of stress.The portion of the blank between the die wall and punch surface is subjected to pure tension, whereas the portion lower down near the bottom is subject both to tension and bending. The portion of metal blank, which forms the flange at the top of the cup is subjected to circumferential compressive stress and buckling and becomes thicker as a result thereof. The flange has therefore to be held down by a pressure pad, otherwise, its surface will become buckled and uneven like an orange peel.Deep drawing is a difficult operation and the material used should be specially malleable and ductile, otherwise it will crack under the induced stresses. The wall thickness of a deep drawn component does not remain uniform. The vertical walls become thinner due to tensile stresses. But the thinnest portion is around the bottom corner of the cup all around. This thinning of sheet at these locations is called necking.After deep drawing, the component may be subjected to certain finishing operations like Ironing,The object of which is to obtain more uniform wall thicknessIroning

Ironing to achieve a more uniform wall thickness in a drawn cup: Start of process; (2) during process. Note thinning and elongation of walls. Symbols v and F indicate motion and applied force, respectivelyENGINEERING ANALYSIS OF DRAWINGAnother way to characterize a given drawing operation is by the reduction r, where

It is very closely related to drawing ratio. Consistent with the previous limit on DR(DR 2.0), the value of reduction r should be less than 0.50.A third measure in deep drawing is the thickness-to-diameter ratio t/Db (thickness of the starting blank t divided by the blank diameter Db). Often expressed as a percentage, it is desirable for the t/Db ratio to be greater than 1 %.

Wrinkling in the flangeSHEET-METAL OPERATIONS NOT PERFORMED ON PRESSESA number of sheet-metal operations are not performed on conventional stamping presses. In this section we examine several of these processes: Stretch Forming, Spinning, High-energy-rate Forming Processes.STRETCH FORMINGStretch forming is a sheet-metal deformation process in which the sheet metal is intentionally stretched and simultaneously bent in order to achieve shape change.The process is illustrated as shown in Figure for a relatively simple and gradual bend.

Stretch forming: (1) start of process; (2) form die is pressed into the work with force F die, causing it to be stretched and bent over the form. F =stretching force.The work part is gripped by one or more jaws on each end and then stretched and bent over a positive die containing the desired form.The metal is stressed in tension to a level above its yield point.When the tension loading is released, the metal has been plastically deformed. The combination of stretching and bending results in relatively little spring back in the part.An estimate of the force required in stretch forming can be obtained by multiplying the cross-sectional area of the sheet in the direction of pulling by the flow stress of the metalIn equation form F=L t Yf where F=stretching force, N., L = length of the sheet in the direction perpendicular to stretching, mm. t = instantaneous stock thickness, mm.Yf = flow stress of the work metal, Mpa.The die force Fdie shown in the figure can be determined by balancing vertical force components

SPINNINGSpinning is a metal-forming process in which an axially symmetric part is gradually shaped over a mandrel or form by means of a rounded tool or roller.The tool or roller applies a very localized pressure (almost a point contact) to deform the work by axial and radial motions over the surface of the part.Basic geometric shapes typically produced by spinning include cups, cones, hemispheres, and tubes.There are three types of spinning operations: Conventional Spinning Shear Spinning Tube Spinning.Conventional Spinning

Conventional spinning:(1) setup at start of process;(2) during spinning; and(3) completion of process.Conventional spinning is the basic spinning operation.As illustrated in Figure, a sheet-metal disk is held against the end of a rotating mandrel of the desired inside shape of the final part, while the tool or roller deforms the metal against the mandrel.The process requires a series of steps, as indicated in the figure, to complete the shaping of the part. The tool position is controlled either by a human operator, using a fixed fulcrum to achieve the required leverage, or by an automatic method such as numerical control.These alternatives are manual spinning and power spinning. Power spinning has the capability to apply higher forces to the operation, resulting in faster cycle times and greater work size capacity.Shear Spinning

Shear spinning: (1) setup (2) completion of process.In shear spinning, the part is formed over the mandrel by a shear deformation process in which the outside diameter remains constant and the wall thickness is therefore reduced, as in Figure.This shear straining (and consequent thinning of the metal) distinguishes this process from the bending action in conventional spinning.Several other names have been used for shear spinning, including flow turning, shear forming, and spin forging. The process has been applied in the aerospace industry to form large parts such as rocket nose cones.For the simple conical shape in our figure, the resulting thickness of the spun wall can be readily determined by the sine law relationship: tf =t sin Where tf = The final thickness of the wall after spinning. t = The starting thickness of the disk. = The mandrel angle (actually the half angle).Thinning is sometimes quantified by the spinning reduction r:

There are limits to the amount of thinning that the metal will endure in a spinning operation before fracture occurs.

Tube SpinningTube spinning is used to reduce the wall thickness and increase the length of a tube by means of a roller applied to the work over a cylindrical mandrel, as in Figure Tube spinning is similar to shear spinning except that the starting work piece is a tube rather than a flat disk.The operation can be performed by applying the roller against the work externally (using a cylindrical mandrel on the inside of the tube) or internally (using a die to surround the tube).It is also possible to form profiles in the walls of the cylinder, as in Figure (c), by controlling the path of the roller as it moves tangentially along the wall.

Tube spinning: (a) External; (b) Internal; and (c) Profiling.HIGH-ENERGY-RATE FORMING(HERF)Several processes have been developed to form metals using large amounts of energy applied in a very short time. Owing to this feature, these operations are called high-energy-rate forming (HERF) processes.They include Explosive forming. Electro-hydraulic forming.Electromagnetic forming.Explosive formingExplosive forming involves the use of an explosive charge to form sheet (or plate) metal into a die cavity.One method of implementing the process is illustrated in Figure

Explosive forming: (1) setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface.The work part is clamped and sealed over the die, and a vacuum is created in the cavity beneath.The apparatus is then placed in a large vessel of water.An explosive charge is placed in the water at a certain distance above the work.Detonation of the charge results in a shock wave whose energy is transmitted by the water to cause rapid forming of the part into the cavity.The size of the explosive charge and the distance at which it is placed above the part are largely a matter of art and experience.Explosive forming is reserved for large parts, typical of the aerospace industry.Electro-hydraulic FormingElectro-hydraulic forming is a HERF process in which a shockwave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in a transmission fluid (water).Owing to its principle of operation, this process is also called electric discharge forming.

Electro-hydraulic forming setup.Electrical energy is accumulated in large capacitors and then released to the electrodes.Electro hydraulic forming is similar to explosive forming.The difference is in the method of generating the energy and the smaller amounts of energy that are released.This limits electro-hydraulic forming to much smaller part sizes.Electromagnetic FormingElectromagnetic forming, also called Magnetic pulse forming, is a process in which sheet metal is deformed by the mechanical force of an electro-magnetic field induced in the work part by an energized coil.The coil, energized by a capacitor, produces a magnetic field.This generates eddy currents in the work that produce their own magnetic field.The induced field opposes the primary field, producing a mechanical force that deforms the part into the surrounding cavityDeveloped in the 1960s, electromagnetic forming is the most widely used HERF process .it is typically used to form tubular parts.No Punch Needed --- Lower tooling costs.No Static forces --- No Large, Costly press.

Fig: Electromagnetic forming: (1) setup in which coil is inserted into tubular work part surrounded by die; (2) formed partRUBBER PAD FORMING PROCESSESThe two operations discussed in this article are performed on conventional presses, but the tooling is unusual in that it uses a flexible element (made of rubber or similar material) to effect the forming operation.The operations are The Guerin process.Hydro forming.Guerin ProcessThe Guerin process uses a thick rubber pad (or other flexible material) to form sheet metal over a positive form block, as in Figure.

Guerin process: (1) before and (2) after. Symbols v and F indicate motion and applied force, respectively.The rubber pad is confined in a steel container.As the ram descends, the rubber gradually surrounds the sheet, applying pressure to deform it to the shape of the form block.It is limited to relatively shallow forms, because the pressures developed by the rubberup to about 10 Mpa. are not sufficient to prevent wrinkling in deeper formed parts.The advantage of the Guerin process is the relatively low cost of the tooling.The form block can be made of wood, plastic, or other materials that are easy to shape, and the rubber pad can be used with different form blocks.These factors make rubber forming attractive in small-quantity production, such as the air craft industry, where the process was developed.Hydro-formingHydro-forming is similar to the Guerin process; the difference is that it substitutes a rubber diaphragm filled with hydraulic fluid in place of the thick rubber pad.

Hydroform process: (1) start-up, no fluid in cavity; (2) press closed, cavity pressurizedwith hydraulic fluid; (3) punch pressed in to work to form part. Symbols: v=velocity, F=applied force, P=hydraulic pressureThis allows the pressure that forms the work part to be increasedto around 100 Mpa. thus preventing wrinkling in deep formed parts.In fact, deeper draws can be achieved with the hydroform process than with conventional deep drawing.This is because the uniform pressure in hydro-forming forces ,the work to contact the punch throughout its length, thus increasing friction and reducing the tensile stresses that cause tearing at the base of the drawn cup.ALTERNATIVE PRESSING AND SINTERING TECHNIQUESThe conventional press and sinter sequence is the most widely used shaping technology in powder metallurgy. Additional methods for processing PM parts are discussed in this section.ISOSTATIC PRESSINGA feature of conventional pressing is that pressure is applied uniaxially.This imposes limitations on part geometry, because metallic powders do not readily flow in directions perpendicular to the applied pressure.Uniaxial pressing also leads to density variations in the compact after pressing.In Isostatic Pressing, pressure is applied from all directions against the powders that are contained in a flexible mold; hydraulic pressure is used to achieve compaction.Isostatic pressing takes two alternative forms: Cold isostatic pressing (CIP).Hot isostatic pressing (HIP).Cold Isostatic Pressing(CIP)

Cold isostatic pressing: (1) powders are placed in the flexible mold; (2) hydrostatic pressure is applied against the mold to compact the powders; and (3) pressure is reduced and the part is removed.Cold Isostatic Pressing(CIP)CIP involves compaction performed at room temperature. The mold, made of rubber or other elastomer material, is oversized to compensate for shrinkage.Water or oil is used to provide the hydrostatic pressure against the mold inside the chamber.The processing sequence in cold isostatic pressing as shown in fig.Advantages of CIP include more uniform density, less expensive tooling, and greater applicability to shorter production runs.Good dimensional accuracy is difficult to achieve in isostatic pressing because of the flexible mold.Consequently, subsequent finish shaping operations are often required to obtain the required dimensions, either before or after sintering.

Hot Isostatic Pressing(HIP)HIP is carried out at high temperatures and pressures, using a gas such as argon or helium as the compression medium.The mold in which the powders are contained is made of sheet metal to withstand the high temperatures.HIP accomplishes pressing and sintering in one step.Despite this apparent advantage, it is a relatively expensive process and its applications seem to be concentrated in the aerospace industry.PM parts made by HIP are characterized by high density (porosity near zero), thorough interparticle bonding, and good mechanical strength.Thank you